1. Field of the Invention
This invention relates to a process for dewaxing a hydrocarbon feedstock and particularly to a process for dewaxing using a catalyst containing, in combination, a zeolite with Constraint Index greater than 1, a second, different catalyst component and a hydrogenation component.
2. Discussion of the Prior Art
Refining suitable petroleum crude oils to obtain a variety of lubricating oils which function effectively in diverse environments has become a highly developed and complex art. Although the broad principles involved in refining are qualitatively understood, the art is encumbered by quantitative uncertainties which require a considerable resort to empiricism in practical refining. Underlying these quantitative uncertanties is the complexity of the molecular constitution of lubricating oils. Because lubricating oils for the most part are based on petroleum fractions boiling above about 450.degree. F. (230.degree. C.), the molecular weights of the hydrocarbon constitutents are high and these constituents display almost all conceivable structure types. This complexity and its consequences are referred to in well-known treatises, such as, for example, "Petroleum Refinery Engineering", by W. L. Nelson, McGraw-Hill Book Company, Inc., New York, NY, 1958 (Fourth Edition).
In general, the basic premise in lubricant refining is that a suitable crude oil, as shown by experience or by assay, contains a quantity of lubricant stock having a predetermined set of properties, such as, for example, appropriate viscosity, oxidation stability, and maintenance of fluidity at low temperatures. The process of refining to isolate that lubricant stock consists of a set of subtractive unit operations which removes the unwanted components. The most important of these unit operations include distillation, solvent refining and dewaxing, which basically are physical separation processes in the sense that if all the separated fractions were recombined, one would reconstitute the crude oil.
A refined lubricant stock may be used by itself, or it may be blended with another refined lubricant stock having different properties. Or, the refined lubricant stock, prior to use as a lubricant, may be compounded with one or more additives which function, for example, as antioxidants, extreme pressure additives, and VI improvers.
For the preparation of a high grade distillate lubricating oil stock, the current practice is to vacuum distill an atmospheric tower residuum from an appropriate crude oil as the first step. This step provides one or more raw stocks within the boiling range of about 450.degree. (230.degree. C.) to 1050.degree. F. (565.degree. C.). After preparation of a raw stock of suitable boiling range, it is extracted with a solvent, e.g., furfural, phenol, sulfolane, or chlorex, which is selective for aromatic hydrocarbons, and which removes undesirable components. The raffinate from solvent refining is then dewaxed, for example, by admixing with a solvent, such as a blend of methyl ethyl ketone and toluene. The mixture is chilled to induce crystallization of the paraffin waxes, which are then separated from the raffinate. Sufficient quantities of wax are removed to provide the desired pour point for the raffinate.
Other processes, such as hydrofinishing or clay percolation, may be used if needed to reduce the nitrogen and sulfur content or improve the color of the lubricating oil stock.
Viscosity Index (VI) is a quality parameter of considerable importance for distillate lubricating oils to be used in automotive engines and aircraft engines subject to wide variations in temperature. This index indicates the degree of change of viscosity with temperature. A high VI of 100 indicates an oil that does not tend to become viscous at low temperature or become thin at high temperatures. Measurement of the Saybolt Universal Viscosity of an oil at 100.degree. (38.degree. C.) and 210.degree. F. (100.degree. C.), and referral to correlations, provides a measure of the VI of the oil. For purposes of the present invention, whenever VI is referred to, it is meant the VI as noted in the Viscosity Index tabulations of the ASTM (D567), published by ASTM, 1916 Race Street, Philadelphia, PA, or equivalent.
Catalytic techniques are now available for dewaxing of petroleum stocks. A process of that nature developed by British Petroleum is described in The Oil and Gas Journal, dated Jan. 6, 1975, at pages 69-73. See also U.S. Pat. No. 3,668,113. The dewaxing mechanism of catalytic hydrodewaxing is different from that of solvent dewaxing, resulting in some differences in chemical composition. Catalytically dewaxed products produce a haze on standing at 10.degree. F. (-12.degree. C.) above specification pour point for more than twelve hours--known as the Overnight Cloud (ONC) formation. The extent of this ONC formation is less severe with solvent dewaxed oils. Although such an ONC formation does not affect the product quality of catalytically dewaxed oils, it is beneficial to reduce the Overnight Cloud (ONC) formation, since in some areas of the marketplace any increase in ONC is considered undesirable.
U.S. Pat. No. Re. 28,398 describes a process for catalytic dewaxing with a catalyst comprising zeolite ZSM-5. Such a process combined with catalytic hydrofinishing is described in U.S. Pat. No. 3,894,938.
U.S. Pat. No. 3,755,138 describes a process for mild solvent dewaxing to remove high quality wax from a lube stock, which is then catalytically dewaxed to specification pour point.
U.S. Pat. No. 4,053,532 is directed towards a hydrodewaxing operation involving a Fischer-Tropsch synthesis product utilizing ZSM-5 type zeolites.
U.S. Pat. No. 3,956,102 is connected with a process involving the hydrodewaxing of petroleum distillates utilizing a ZSM-5 type zeolite catalyst.
U.S. Pat. No. 4,247,388 describes dewaxing operations utilizing ZSM-5 type zeolites of specific activity.
U.S. Pat. No. 4,222,855 describes dewaxing operations to produce lubricating oils of low pour point and of high VI utilizing zeolites including ZSM-23 and ZSM-35.
U.S. Pat. No. 4,372,839 describes a method for dewaxing crude oils of high wax content by contacting the oils with two different zeolites, e.g., ZSM-5 and ZSM-35.
U.S. Pat. No. 4,419,220 describes a dewaxing process utilizing a Zeolite Beta catalyst.
Copending U.S. patent application, Ser. No. 614,072, filed May 25, 1984, describes a dewaxing catalyst comprising, in combination, a medium pore zeolite and Zeolite Beta in the presence of a hydrogenation component.
The entire contents of these patents and patent application are incorporated herein by reference.
Thus, there are catalysts in the prior art useful for dewaxing processes which result in the production of lubricating oils of enhanced properties. However, improvements in the dewaxing process can be made by utilizing the catalyst of the present invention. These catalysts improve product quality and selectivity for dewaxing processes and hydrocracking.
It is an object of the present invention to provide an improved process for dewaxing a hydrocarbon feedstock.
It is additionally an object of the present invention to provide an improved catalyst having a bifunctional shape-selective hydroisomerization/hydrocracking activity.
It is another object of the present invention to provide improved dewaxing activity, increased product yields, and improved product qualities over that found with existing catalysts, alone or in combination with other catalysts.
These and other objects are fulfilled by the present invention, which is disclosed below.